Thickness probes, such as ultrasonic thickness probes, are commonly used to investigate the properties of machined components. A known method of calibrating such probes for thickness measurement is to use a block of material 100 with one or more blind holes 102 drilled into it, as shown in FIG. 1 (PRIOR ART). The hole diameter and hole depth may vary between the holes in order to provide a range of calibration features. The probe to be calibrated 104 is used to measure the holes from the opposite side of the block. The measured values of material thickness t between the bottom of each hole 102 and the block face 106 are then compared to established thickness values, obtained using a different method, in order to calibrate the thickness probe 104.
When using a thickness probe to investigate a component, the probe is supported by a mechanical holding arm, allowing measurements to be taken at various points on the surface of the component. The position of the holding arm can be controlled using appropriate software, and the arm may be indexed to various positions within a three dimensional coordinate space, including a nominal origin of that space. The same mechanical arm is typically used to support several different probes during examination of a single component. For example, a touch probe is often used in conjunction with an ultrasonic thickness probe in order to map a component. Physical differences between different types of probe mean that a thickness probe mounted in a holding arm will not contact a component at exactly the same point as, for example, a touch probe mounted with the arm in the same position. It is not therefore possible to record the true coordinate position of the thickness probe with respect to the holding arm, or to the reference coordinate space within which it operates, without conducting further investigation to establish this position. Thickness probes are therefore rarely used in applications where this information is required.
When it is required to know the true coordinate position of a thickness probe, one method of calibrating the position of the probe, establishing the offset between the holding arm (whose exact coordinate position is known) and the thickness probe, would be to search over an area of the block to find the minimum measured thickness of the block. This minimum thickness would correspond to the location of the deepest hole. However, several disadvantages are associated with this method of calibrating the spatial position of a probe. The accuracy of this method depends upon the relative sizes of the probe and the calibration holes. A large diameter hole will provide a clear thickness reading but an inaccurate indication of position, as the minimum thickness reading will be obtained at multiple positions within the hole. In contrast, a smaller diameter hole, although reducing the area over which a minimum reading will be obtained, provides only a poor signal and can be difficult to locate. The accuracy of calibration is not therefore improved by using a small diameter hole. In addition to the problems associated with accuracy, the method outlined above is time consuming, as the only way to locate the desired hole is by trial and error, taking a large number of readings in different positions until the position of the hole is determined.